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Human ovaries tend to shut down by age 50 or even younger, yet women commonly live on healthily for decades. This flies in the face of evolutionary theory that losing fertility should be the end of the line, because once breeding stops, evolution can no longer select for genes that promote survival.

The most popular explanation, the “grandmother hypothesis,” argues that a generous post-reproductive life span makes sense if a grandmother improves the survival and reproduction of her grandchildren…

“The problem is that these grandmother benefits aren’t big enough to ever favor stopping breeding between the ages of 40 and 50,” says Michael Cant, an evolutionary biologist at the University of Exeter in England and co-author of a new study on the genesis of menopause published this week in Proceedings of the National Academy of Sciences USA. “When you look at data from hunter-gatherers and other natural fertility populations, the sums just don’t add up.” Grandmothers do benefit their descendants, he says, but the genetic payoff is small compared with those of producing another child.

Cant and co-author Rufus Johnstone, an evolutionary biologist at the University of Cambridge in England, used game theory to argue that menopause is early cessation of reproduction that originated through reproductive conflict between generations. In most cooperatively breeding species, reproduction is suppressed in younger females, who act as helpers to older reproducing females. By contrast, they say, younger women in human social groups win the reproductive sweepstakes, because the older ones stop having babies.

“We showed that, compared to other primates that exhibit a post-reproductive life span, humans really stand out, because there is absolutely no overlap in reproduction between generations,” Cant says. “Women stop breeding on average when the next generation starts to breed.”

This makes evolutionary sense, Cant and Johnstone say, because, contrary to most mammals, young women tend to move to their mates’ communities, where they become immigrants whose only genetic kin are their own children. There is no genetic profit in helping their mothers-in-law bear more children, because they will not share any genes with those children. But an older woman who helps her son’s wife reproduce will benefit by bequeathing 25 percent of her genes to her grandchildren.